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The cells of an animal or higher plant have two sets of chromosomes: they are diploid. When gametes (sex cells) are produced, they have only one set of chromosomes: they are haploid. They have undergone a process of cell division called meiosis. Two things happen during meiosis, each of which makes the offspring more variable. That means they are different from the parents and from each other.

Assortment is when the double set of chromosomes becomes a single set in each gamete. In each pair of chromosomes, which one goes into a single gamete is random. Because the gene alleles on each chromosome are not always the same, this means that there is genetic variation between gametes. This process was Mendel's 'first law', the law of segregation.

Because crossing over occurs during meiosis, this increases the variety of the chromosomes. This makes it possible to get recombination.

The consequence of assortment and crossing over makes it certain that no two offspring of the same mother and father are identical, except for identical twins. They are identical genetically because they developed from the same fertilised egg.

There are advantages and disadvantages of sexual reproduction, compared to asexual reproduction. The main issues are:

Advantages: More variation assists with survival. It increases the chance that at least some offspring of a parent survive. To give an example, suppose a deadly infection occurs in the population. Greater variety increases the chance that some of the population will survive.

Disadvantages: Requires two parents. So, supposing the total number of eggs to be the same, a population reproducing sexually would produce only half as many offspring as a population reproducing asexually.

3. Gastrulation: Blastula continues to grow, cells continue reproducing using mitosis. Several hundred cells on 1 side begin to move in and form a 2 layered embryo. Developed into 3 layered embryo which is endoderm, mesoderm, and ectoderm.